®
ACT4065A
Rev 0, 23-Apr-12
frequency. If RCOMP is less than 15kꢀ, the equation
for CCOMP is:
Stability compensation
Figure 2:
− 5
1 .8 × 10
Stability Compensation
C COMP
=
(10)
(F)
R COMP
If RCOMP is limited to 15kꢀ, then the actual cross
over frequency is 6.1/ (VOUTCOUT). Therefore:
CCOMP = 1.2 ×10 5 VOUT COUT
(F)
(11)
STEP 3. If the output capacitors ESR is high enough
to cause a zero at lower than 4 times the cross over
frequency, an additional compensation capacitor
C
COMP2 is required. The condition for using CCOMP2 is
required. The condition for using CCOMP2 is:
c: CCOMP2 is needed only for high ESR output capacitors
The feedback system of the IC is stabilized by the
components at COMP pin, as shown in Figure 2.
The DC loop gain of the system is determined by
the following equation:
−6
⎛
⎞
1.1×10
⎜
⎜
⎟
⎟
RESROUT ≥ Min
,0.012VOUT
(Ω)
(12)
(13)
COUT
⎝
⎠
And the proper value for CCOMP2 is:
0 .808 V
AVDC
=
AVEA GCOMP
(4)
(5)
(6)
COUT RESROUT
IOUT
CCOMP 2
=
RCOMP
The dominant pole P1 is due to CCOMP
:
Though CCOMP2 is unnecessary when the output ca-
pacitor has sufficiently low ESR, a small value
COMP2 such as 100pF may improve stability against
GEA
fP1
=
C
2πAVEA CCOMP
PCB layout parasitic effects.
The second pole P2 is the output pole:
Table 2 shows some calculated results based on the
compensation method above.
IOUT
fP 2
=
2πVOUT COUT
Table 2:
The first zero Z1 is due to RCOMP and CCOMP
:
Typical Compensation for Different Output volt-
ages and Output Capacitors
1
(7)
fZ1
=
2πRCOMP CCOMP
c
VOUT
2.5V
3.3V
5V
COUT
RCOMP CCOMP CCOMP2
And finally, the third pole is due to RCOMP and
CCOMP2 (if CCOMP2 is used):
22μF Ceramic
22μF Ceramic
22μF Ceramic
47μF SP Cap
47μF SP Cap
47μF SP Cap
12kꢀ
12kꢀ
15kꢀ
15kꢀ
15kꢀ
15kꢀ
2.2nF
1.5nF
2.2nF
1.5nF
1.8nF
2.7nF
1.5nF
2.2nF
2.7nF
None
None
None
None
None
None
47pF
47pF
47pF
1
fP3
=
(8)
2πRCOMP CCOMP2
Follow the following steps to compensate the IC:
STEP 1. Set the cross over frequency at 1/5 of the
2.5V
3.3V
5V
switching frequency via RCOMP
:
2.5V 470µF/6.3V/30mꢀ 15kꢀ
3.3V 470µF/6.3V/30mꢀ 15kꢀ
2πVOUT COUT fSW
10 GEA GCOMP × 0.808 V
RCOMP
=
5V
470µF/10V/30mꢀ 15kꢀ
8
(9)
(Ω)
= 2 .75 × 10 VOUT C OUT
c: CCOMP2 is needed only for high ESR output capacitors
but limit RCOMP to 15kꢀ maximum.
Figure 3 shows a sample ACT4065A application
circuit generating a 2.5V/2A output.
STEP 2. Set the zero fZ1 at 1/4 of the cross over
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